1. Field of the Invention
The present invention refers to a device for calibrating the frequency of an oscillator, to a phase looked loop circuit comprising said calibration device and to the related frequency calibration method.
2. Description of the Related Art
In the present radio communication systems phase looked loop circuits (PLL) are generally utilized for generating oscillating electric signals. Generally the PLLs comprise a voltage controlled oscillator (VCO) generation an oscillating output signal as a response of a voltage level applied at the input. The specific parameters of the voltage/frequency relationship depend on the parameters of the VCO manufacturing process, the values of the electric components that it comprises, the ambient temperature and the other factors.
For reducing the effects of the temperature variations and of the process parameters, devices for calibrating the frequency of the VCO are presently used.
Such devices become very important for the following reasons.
The reduction of the sizes of the integrated circuits which are due to the generation of MOS transistors having smaller and smaller channels imposes a reduction of the supply voltage and, for this reason, a reduction of the input voltage of the VCO.
The need of increasing the rejection of the spur radiation emissions in PLL and the risk of frequency variations of the VCO which are due to the interferences given by the coupling with the rest of the circuitry when a complex system is formed on a chip, imposes a reduction of the gain of the VCO.
A PLL having a device for calibrating a frequency of a VCO is disclosed in the article “A Fully Integrated PLL Frequency Synthesizer LSI for Mobile Communication System.”, 2001 IEEE Radio Frequency Integrated Circuits Symposium, pages 65–68. The PLL device comprises a programmable divider, a phase comparator, a filter, a delta-sigma converter, a VCO and a frequency error detector. The frequency band of the VCO is selected by means of a digital control signal and the resonant circuit of the VCO has N frequency bands of operation. When new frequency data are sent to the PLL the VCO has an input constant voltage. As a response of a starting digital signal one among the N bands of the resonator circuit is selected and the VCO oscillates at a frequency f1 that is sent to the programmable divider having at the input the output signal of the sigma-delta converter; the output signal of the programmable divider is the signal fdiv. The error detector compares the frequency fdiv with a reference frequency fref and it generates a new digital signal for selecting another frequency band of the VCO. The process is iterated several times until the frequency difference between the signal fdiv and the signal fref becomes less than a prefixed value. When the corrected frequency band of the VCO has been determined, the VCO does not have any longer at the input the constant signal but the output signal of the filter so that the PLL acts in closed loop condition.
The precision of the frequency calibration in this device is due to the bit number of the digital signal; however the use of a digital signal with a large bit number increases the convergence time of the calibration method. It is therefore necessary a trade-off between the bit number of the digital signal and the convergence time of the calibration method.